Thermistor and method of heat-treating it



H. C. COLPITTS THERMISTOR AND METHOD OF HEAT TREIIINGy IT Dec. 29, 1953Filed June 15, 1951 Patented Dec. 29, 1953 V"IHERMISTGR. .AND METHOD rOF"HEAT-TREATI'NG IT 'HarryC'..Colpitts, Montreal, Quebec, Canada, as-

signor to Northern Electric Company, Limited, -MontreaL Quebec, Canada,a corporation of Canada Application June 15, 1951,"Seri'al No. 231,814

- 5' claims.

This invention relates 6to -thermistors and more particularly toy amethod` of heat treating the thermistor element.

Thermistors, or semi-conductors having a negativeresistance-.temperature coeiilcient, have been known for several years.Their value as possible circuitl elements has long been recognized.I-Iowever, one of the reasons for the slow adoption'fof thermistors intogeneral use has been a diiiculty experienced With all semi-conductor'elements, that is, the diiiiculty of obtaining uniform characteristics.

It is the object of this invention to provide a simplified method forthe manufacture of thermistors. v

It is the further object of this invention to provide a method forproducing thermistors of uniform resistance.

It is the further object of this invention to provide means for makingan adjustment of the cold resistance after the thermistor element has"been assembled in a protective envelope.v

It is the further object of this invention to provide a method ofreclaiming thermistors whose resistance characteristics have `changeddue to aging or abnormal use.

An understanding of the invention may be had by reference to thedrawings in which:

Fig. l illustrates a string of thermistor beads.

Fig. 2 illustrates an individual bead cut from the string.

Fig. 3 shows the thermstor element mounted in a protective envelope.

Fig., 4 shows the resistance distribution of a group of thermistorbeads.

Fig. 5 is a charge showing the relation between heating current and beadtemperature.

The present art used in the manufacture of bead type thermistors isillustrated in Fig. 1 in which a string of small spheroidal beads l2 ofsemi-conductor material is formed on a pair of lead wires ll whichsupport the bead and make electrical contact with it. To do this thepair of lead wires I I, which may be of platinum alloy, are strung on asuitable support. The lead wires used to form the string of Fig. V1 maybe about twelve inches in length, four-thousandths of. an inch indiameter, and spaced one thirty-second of an inch apart. Thesemi-conductor material is ground into a paste with avolatilevehiclesuch as distilled water. Several kinds oi"V semi-.conductormaterials are known to the art, of which the oxides of cobalt, iron,nickel, zinc, manganese, and uranium have been widely used. A ceramicbinder. 91 ller may A be included in.theepaste` 2 e Smalldaubs ofthepaste are placed on theparallel wiresat quarter inch intervals. Surfacetensionpulls. the paste into a spheroidal shape.

v*The rbeads are dried by evaporating the vehicle,. hardened by passingthe string of beads shown in Fig. 1 through a small flame, and thensintered by passing through furnace. Individual beads illustrated inFig. 2 are then cut from the string and then tested. Beads having thedesired resistance characteristic are selected and mounted in anevacuated glass envelope 1.3,

vgiving following'information on the changing of resistance ofthermistors no mention will bev made of delay-time although it must bekept'in mind that the delay-time is a function of the re,- sistance.

In the ring process described above, it isy necessary to control thefurnace temperature. the speed with which the string of beads is pulledthrough the furnace, the rate of cooling, and the amount of air passingthrough the furnace. n

spite of these controls it is diflicult `to obtain a' uniform resistancecharacterisie of the bead.

, The bead and wire assembly is fragile, and is easily subject tomechanical change. Handling of the assembly tends to increase theresistancev of the bead. Since it is necessary to handle the beadseveral times before assembly in the glass` envelope is complete, beadsvwhich have a desired characteristic after firing may be spoiled orchanged during assembly. ln addition, differ--l ences in individualbeads cannot be corrected..V

since ring a string does not permit of individual adjustment.

Fig. 4 shows a representative distribution of cold resistance values for-a group of thermistor beads made according to the ypresent art andtested` while in the stage illustrate-d in Fig. 2. The horizontal axisof Fig. 4 is in thousands ofohms, while the vertical axis is in percent' of the total group for a class intervalfof 5,000 ohms.Y

Thusthe point P indicates that 10 per cent of-a representative group ofthermistor beads would have a cold resistance in the range 45,000 ohmsto 3 50,000 ohms. From Fig. 4, it is apparent that the yield ofthermistors lying within practical resistance limits of 125 per cent isvery small. As mentioned above, this is aggravated by further handlingof the bead after selection at this stage.

In the method according to this invention, the beads are made in stringsas shown in Fig. 1 and passed through the sintering furnace. Theindividual beads of Fig. 2 are then cut from the string and completelyasse-mbled. After assembly, the resistance of the thermistor beads areindividually adjusted to the required value by heat treatment. Since thebead is protected by envelope i3, further handling does not alter thecharacteristics.

According to this invention, the resistance of uranium oxide beads maybe adjusted by heat treatment at temperatures from 600 degreescentigrade to 1100 degrees centigrade. This range is well below theinitial sintering temperature which may be 1300 degrees centigrade. Ithas been found that beads which have a resistance below the average of agroup may be brought nearer the average by heat treatment at the lowerend of the temperature range 600-1100 degrees centigrade. Furthermore,beads which have a resistance above the average of a group mty bebrought nearerthe average by heat treatment at the upper end of thetemperature range 600-1100 degrees centigrade.

According to this invention, heating may be accomplished by the passageof a current many times the working current through the bead. Fig. 5shows a representative relation between surface temperature and heatingcurrent for uranium oxide-pyrex beads il@ inch in diameter. The normalworking current for such beads may be a few milliamperes. Quantitiesexpressed in the discussion below rre for thermistor beads oi this type.

If the bead resistance is above the average of the group, it may belowered by the momentary application of a heating current ofapproximately 0.7 ampere. Generally speaking, the larger the heatingcurrent (or the higher the temperature), the gre: ter the drop inresistance. However, currents above one ampere yield inconsistentresults and may damage the bead permanently. The proper value of heatingcurrent may be determined for a group by trial on a few beads. Theresistance may not be lowered indefinitely, but behaves as though itwere rpproaching a stable range of values. The beads, or completedthermistors are allowed to cool in air.

If the bead resistance is below the average of the group, it may beincreased by successive momentary applications ci a heating current of2pproxirnateli7 0.3 ampere. Generally speaking, increasing the number ofhot and cold cycles by repeated applications of heating current followedby a cooling period increases the resistance of the bead. The resistrncemay not be increased indeinitely, but behaves as though it wereapproaching a stable range of values.

In general, beads which are above the group average for resistance arenot affected by heat trerting at the lower end of the range 600-1100degrees centigrade, and beads which are below the group average are notaiiected by heat treatment at the upper end of the range 60G-1100degrees centigrade.

The heat treating temperatures mentioned above are surface tempersturesestimated by means of an optical pyrometer. It will be appreciated thatthe interior temperature of the bead may be considerably above thesevalues.

Thermistor beads which have changed values due to aging or overloadingmay be readjusted by the method outlined above.

It is convenient to use a relatively high voltage source of poorregulation to supply the heating current. Thus a 70 volt supply workingthrough a 3500 ohm resistor which may be variable has been foundsuitable for the heat treatment outlined above.

The behavior of thermistors of initially high resistance is illustratedin the Table I below in which the resistance is expressed in thousandsof ohms, and the heating current in amperes:

TABLE I Initial Eat rea lllgW-l a Thermistor sample cold re current 0fsistance The behavior of thermistors of initially low resistance isillustrated in Table II below in which the resistance is expressed inthousands of ohms:

Cold resistance after hefaieng with a cureit Initial o ampere an aThermistor sample cold relowmg to e001 sistance Three Once Twice timesWhat is claimed is:

l. A method of making uranium oxide thermistors with atemperature-resistance characteristic of high accuracy which comprises apreforming of the semiconductor element and thereafter heat treating bythe momentary dissipation of electric energy in the said element, saidenergy being very large compared to the normal working capacity of thethermistor.

2. A method of making uranium oxide thermistors with atemperature-resistance characteristic of high accuracy which comprises apreforming of the semiconductor element and assembling said element inits protective envelope or holder, and thereafter heat treating the saidelement by the momentary dissipation of electric energy in it, saidenergy being very large compared to the normal working capacity of thethermistor.

3. A method of adjusting the resistance of uranium oxide thermistors toobtain a temperature-resistance characteristic of high accuracy wherebythe semiconductor element is heat treated by the momentary dissipationin it of electric energy, said energy being very large compared to thenormal working capacity of the thermistor. K

4. A method of making uranium oxide thermistors with atemperature-resistance characteristic of high accuracy whereby theresistance is adjusted according to the method of claim 3 after thesemiconductor element has been assembled into its protective envelopeor. holder. References Cited in the le of this patent 5. A method ofmaking thermistors with a UNITED STATES PATENTS temperature-res1stancecharacteristic of high accuracy having a semiconductor element con-Number Name Date taining uranium oxide in which said element is 5119061853 Hedlger May 2, 1933 heat treated in the range 600 to 1100degrees 19u15 Randolph Apr- 16, 1940 centigrade by the momentarydissipation in the 2,271'975 Y Hau Feb- 31 1942 said element of electricenergy.

HARRY C. COLPITTS.

1. A METHOD OF MAKING URANIUM OXIDE THERMISTORS WITH ATEMPERATURE-RESISTANCE CHARACTERISTIC OF HIGH ACCURACY WHICH COMPRISES APREFORMING OF THE SEMICONDUCTOR ELEMENT AND THEREAFTER HEAT TREATING BYTHE MOMENTARY DISSIPATION OF ELECTRIC ENERGY IN THE SAID ELEMENT, SAIDENERGY BEING VERY LARGE COMPARED TO THE NORMAL WORKING CAPACITY OF THETHERMISTOR.